Field of Invention
This invention relates to a substrate structure, and more particularly, to a multilayer substrate which reduces the residual stress induced by thermal expansion mismatch in electronic packages.
Description of Related Arts
Multilayer metallized substrates have been extensively used in electronic packages, such as DBC (direct bonded copper) substrate, AMB (active metal braze), DPC (direct plated copper), thick film plated up technology, and thin film technology. As an example, shown in FIG. 1, a typical DBC substrate consists of a ceramic base material 10 and copper metallization 11 on one side or on both sides. The copper metallization 11 is plated with a nickel plating layer 12, and a gold plating layer 13. Commonly used base materials are ceramics such as AlN, Al2O3, BeO, and Si3N4. The ceramic provides mechanical strength and electrical insulation from a live circuit to chassis, while the copper metallization provides interconnections to form an electrical circuit. The nickel plating layer 12, which separates the copper metallization 11, is formed on top of the copper metallization 11. The gold plating layer 13 is formed on top of the nickel plating layer 12. The nickel plating layer 12 between the copper metallization 11 and the gold plating layer 13 serves as a diffusion barrier which prevents the diffusion between the copper metallization 11 and the gold plating layer 13. On top of the gold plating 13, electronic devices, such as diodes and IGBTs, are attached by solder or conductive epoxy. The gold plating layer 13 prevents oxidation and improves solderability and bondability for die attachment. The combination of all layers makes the electrically insulated substrate strong in mechanical strength, and forms an electrical circuitry when electronic devices are bonded onto the metallization. The substrate is then attached by solder, epoxy, or thermal grease to a metal heat spreader such as copper or aluminum, for heat spreading and dissipating.
A common issue of the abovementioned structure where a multilayer metallized substrate with electronic devices populated is attached onto a metal heat spreader is the excessive residual stress caused by thermal expansion mismatch between the internal layers. The excessive residual stress plays a major role in the long-term reliability of electronic packages which are characterized as multilayered structures. Among the materials of the layers in a multilayer metallized substrate that is attached to a metal heat spreader, the coefficient of thermal expansion (CTE) of silicon is 2.5 ppm/° C., CTE of ceramic materials is normally between 3 ppm/° C. and 10 ppm/° C., CTE of copper is 17 ppm/° C., CTE of nickel is 13 ppm/° C., CTE of gold is 14 ppm/° C., and CTE of solder materials is generally between 13 ppm/° C. to 35 ppm/° C. When temperature changes, the materials of all layers of this multilayer structure expand or contract at different rates because the materials have different CTE's. Temperature range of applications in automobile, military, and aerospace usually falls between −55° C. and 150° C. In some extreme cases, the application temperature can get as low as −65° C. and as high as 200° C. Exposure to such a wide temperature range can result in excessive thermal residual stress as high as several hundred mega Pascal in the multilayered structure, which can induce solder interface cohesive failure, copper/ceramic interface fracture, ceramic cohesive fracture, copper/solder adhesive failure, and even cracking of silicon electronic components. As a result of these failures, the actual contact area between layers of the substrate is reduced, so that the electronic devices that are bonded onto the substrate will run hotter due to a more restricted path of conduction, indirectly reducing the life of the product (or a lower MTBF number). In some severe cases, the assembly can simply malfunction due to the cracking of the ceramic substrate, compromising the integrity of the circuitry that forms on top of the substrate, or due to die cracking as a direct result of substrate cracking.
Several approaches have been utilized to address this long term reliability issue induced by mismatch of thermal expansion. One approach is to use heat spreader materials with similar thermal expansion coefficient as that of ceramics and electronic components. For example, AlSiC and CE alloy have a CTE of less than 10 ppm/° C. The major drawbacks of these materials are the high material cost, and long lead time, which prevent their widespread use in electronic packaging industry, unless very large volume is used for a single design. In addition, AlSiC is a brittle material that is difficult to machine without running into the risk of cracking it, requiring a custom design to incorporate aluminum rich area for each mounting hole so that it can be drilled without cracking.
It is desirable, therefore, to provide a multilayer substrate which reduces the residual stress induced by thermal expansion mismatch in electronic packages.